More pregnant women are increasing the risk of their babies being born prematurely by taking prescription narcotic painkillers.

The painkillers, known as opioids, include drugs such as hydrocodone, oxycodone, codeine and morphine. Researchers found that, of over 112,000 pregnant women from Tennessee, U.S., almost 30% used opioids at least once.

Past research in the U.S. has shown that prescription painkiller use during pregnancy has doubled over the last 15 years.

Opioids could result to premature birth, low birth weight, or potentially painful withdrawal symptoms for the baby after birth. They also found that these risks went up if the pregnant women also smoked.

“I was surprised by the number of women prescribed opioid pain relievers in pregnancy,” said lead author Dr. Stephen Patrick, a neonatologist and assistant professor of pediatrics at Vanderbilt University.For this study published in the journal Pediatrics, researchers found that of the 112,000 women’s medical records on the Tennessee Medicaid program they analyzed, 28% percent have taken at least one painkiller.

42% of these women, compared to the 26% of the women not prescribed painkillers, also smoked during pregnancy. Doing this, they increased the risk of their babies going through withdrawal.For women who are taking these drugs as maintenance, Patrick suggested to avoid smoking to lessen the risk of withdrawal and premature birth.“Premature babies often have developmental delays and neurological problems, so anything you can do to reduce rates of prematurity, including avoiding opioids during pregnancy, you should,” said obstetrician-gynecologist Dr. Jennifer Wu at Lenox Hill Hospital.